I didn't find Trixx to be useful at all. I had this same issue last night and I finally got MSI Afterburner to work which is awesome because I love it. Basically what I did was change the clocks (started by lowering mem clock as low as it would go) then applying settings. Then under settings I changed to the other GPU and did the same thing. Then when I switched back to the original GPU I was able to clock the core up and the mem down some more. Just kept switching back forth until desired. Save profile.....win?

Hope that helps.